Air traffic control (ATC) is used at most airports to coordinate general aircraft traffic around the airport and en route. Traditionally, a pilot uses a radio to speak to an ATC center to request permission or to receive instructions therefrom. With increasing air traffic, the volume of voice communication is straining the capacity of the voice network. Consequently, data-link applications have been developed to provide textual communication between pilots and air traffic controllers.
One of these data-link applications, called Controller Pilot Data Link Communication (CPDLC), provides for the direct exchange of text-based messages between a controller and a pilot. The CPDLC application enables the pilot to communicate electronically with an ATC center by guiding the pilot through a series of screen configurations or displays that either elicit flight information from the pilot or notify the pilot regarding flight information. The CPDLC application may be part of a larger flight information/control software program or may serve as a stand-alone program.
The CPDLC protocol as defined in Eurocae document ED110/RTCA doc 280 requires the pilot to respond to each ground message within 100 seconds. If the pilot needs more time to respond, the pilot has to manually send a “STANDBY” message. The pilot then has 100 more seconds to respond from the time the STANDBY message was sent. If the pilot sends the STANDBY message shortly after receiving the ground message (e.g., 20 s), then the amount of time to respond (20 s+100 s=120 s) is not maximized. The pilot can wait and try to send the STANDBY message just before the 100 second time limit of the response timer expires in order to maximize the time to evaluate the message (e.g., 99 s+100 s=199 s). If the pilot waits just a little too long, however, the response timer will expire and the pilot will not be allowed to respond to the ground message. An air traffic controller is in a similar high pressure situation but with a different time limit of 240 seconds to respond to an aircraft message.